I've just seen a product on DriveThruRPG that's described as a "splash book." I think it's probably just a really funny malapropism of "splatbook."
The story goes that back in the nineties when White Wolf started releasing their Clanbooks and Tribebooks and Kithbooks and what have you for their World of Darkness game lines they were released in a softcover format because as non-core products they were not expected to sell quite as well. These books came to be called "splatbooks" because of the "splat" sound they make on account of their soft flapping covers. Anyway down the line this sort of led to the part of your character that most clearly differentiates them from other characters of their broad supernatural type (Clan for Vampires, Tribe for Werewolves, Kith for Changelings, etc.) being referred to as a character's "splat."
Both splatbook and the term splat as a character-building term are still in use in RPG discussions to an extent, but these days the former can cover pretty much any sourcebook, even if it's in a hardcover format (although if it revolves around a certain character type it's easier to characterize as such: one could argue that 3.5's Complete Warrior was a Warrior splatbook in spite of being a hardcover), and the latter sometimes gets applied outside of World of Darkness and other games of White Wolf descent, although that is where you'll still most commonly find it (the different types of titular exalted in Exalted would count as that game's splats).
Anyway so it's probably called a splash book because they expect that it'll make quite a splash in the world of 5e compatible products based on actual play shows,
I'm going to be introducing the RPG-playing world to the concept of crashbooks: that's right, when you open the PDF it crashes your computer
Historical Revisionism Alert:
The aforementioned origin of "splatbook" is likely to be a case of folk etymology. Supposedly the term actually originates from the books being referred to as *books on newsgroups (the asterisk being used as a wildcard character) and the asterisk being colloquially known as a "splat."
Blue Planet should have had splashbooks, tbh
Huh, I always thought it was a kind of contraction of "supplement" ie SPLT book
















